Some good sources are (these will also be listed in
/usr/share/doc/xplanet/README.Debian)

The Xglobe & Xplanet Maps page
(www.radcyberzine.com/xglobe)

The Ssystem Home Page
(http://socios.las.es/~amil/ssystem/)

JPL's Maps of the Solar System (maps.jpl.nasa.gov)

The Planetary Maps Hub

(www.lancs.ac.uk/postgrad/thomasc1/render/maps.htm)

Saturn map
(http://www.mmedia.is/~bjj/data/saturn/saturn.jpg)

If no image is specified with the -image option, xplanet looks
in the current directory first, and then in the directory specified
by command-line option -mapdir (if used), and finally in the
directory specified at compilation time (/usr/share/xplanet/images
for Debian) for the file to use as the day map. This file should be
named body.extension, where body can be mercury, venus, earth,
moon, mars, jupiter, io, europa, ganymede, callisto, saturn, titan,
uranus, neptune, or pluto. The extension of the file by default is
jpg. After installation, it is suggested that you should get planet
maps from any of above locations and install it in some directory
of your choice, say $HOME/xplanet and use the -mapdir option to
point to the directory that contains the planet day maps.